Central venous catheters are essential for the care of patients requiring chronic venous access, yet clots often develop at the catheter tip and block blood withdrawal. Although heparin flushes are routinely used to prevent this, their is little evidence that they are any more effective than saline. This study compares the effectiveness of standard heparin with flushes of a new anticoagulant, lepirudin, given at least daily for the first 3 - 4 weeks after the catheter has been inserted. The measured parameter is the frequency with which the catheters develop withdrawal occlusion.